This invention relates to enhanced fatigue nuts and improved methods for mechanically fastening enhanced fatigue nuts with threaded mating components.
Mechanical fasteners are well known and there are a wide variety of mechanical fasteners. One such type of mechanical fastener is a threaded connector. For example, bolts and nuts are threaded connectors. Fasteners of this type and other types have a wide range of application. One field in which mechanical fasteners, including threaded fasteners, are employed is the aerospace industry. It is particularly important that mechanical fasteners function properly when used in the aerospace industry and other industries because failure of a mechanical connection has the potential to cause not only operational problems, but catastrophic ones as well. There are a wide variety of ways in which mechanical failure of a fastener may occur. Some of the well documented modes of failure of fasteners include brittle failure, fatigue failure, failure in shear, failure due to temperature effects, failure in tension and failure due to stress corrosion cracking. As those skilled in the art will appreciate, one way to reduce the likelihood of failure by any of these modes is to select the proper material for a given loading condition. Another way is to change the load distribution on the fastener by changing the fastener's configuration.
This invention is related to a particular type of fastener, a nut, and to threaded fastening systems employing it. In accordance with this invention, the enhanced fatigue nut is designed to increase the resistance to fatigue failure of a threaded mating component to which the enhanced fatigue nut is mated. The term enhanced fatigue nut signifies that the nut of this invention increases the fatigue life or the resistance to fatigue failure of a mating component to which the enhanced fatigue nut is mated. This is the definition of the term enhanced fatigue nut as it is used in this application. Briefly, it should be understood that when a nut is mated with a mating component, the nut is typically in compression and the mating component is typically in tension.
The fatigue strength of a material is the ability of the material to resist dynamic failure when alternating or fluctuating stresses are applied. Fatigue failure generally occurs when a stress below the ultimate strength and perhaps even below the yield strength of the material is applied repetitively or cyclically to a material. Generally, there are two phases of fatigue failure, crack initiation and crack propagation. Crack initiation refers to the formation of a small crack that is typically microscopic in size. This crack may occur due to a manufacturing defect or at a point of discontinuity in the material, such as where a hole or a thread is disposed. Because stresses concentrate at these cracks, these cracks will slowly propagate as the material is repetitively stressed, and the crack propagation phase of fatigue failure is entered. The probability of crack initiation and the rate of crack propagation is proportional to the alternating stress. After a stress has been applied a certain number of times, the crack may propagate relatively rapidly and sudden failure of the material may occur. This type of failure is known as fatigue failure. Because fatigue failure can occur rapidly and without much advanced warning, it is particularly important to prevent fatigue failure of a material that will be loaded in a repetitive or cyclical fashion. Crack initiation is generally a function of the geometry of a material, while crack propagation is generally a function of the type of material from which the component is manufactured.
Resistance to fatigue failure is typically determined by applying an alternating stress, a stress that alternates between a high and a lower level, to a specimen. By applying the alternating stress, the number of cycles until the material fails at that stress may be determined. The stress at which a material fails for a given number of cycles is commonly referred to as the fatigue strength of the material. Tests can be performed for various loading conditions, and a graph of the fatigue strength verses the number of cycles can be developed. From this graph, the fatigue strength for a given number of cycles can be determined. By maintaining the alternating stress below the fatigue strength for a given number of cycles, fatigue failure of the material may be prevented.
Fatigue failures in correctly designed and manufactured aerospace externally threaded components, such as bolts or studs, undergoing axial loading, typically occur in the thread root. It is the alternating component of the maximum principle stress which is responsible for these fatigue failures. Typically, the failure occurs in the thread root that has the maximum alternating maximum principle stress found in any thread root of the externally threaded component. The maximum alternating maximum principle stress will hereafter be referred to as the limiting stress.
An externally threaded component may also have an endurance limit. The endurance limit is the fatigue strength of the material or the limiting stress below which a material will not undergo fatigue failure, no matter how great the number of applied cycles. Graphically, the endurance limit is generally represented as a plateau on a plot of stress verses the number of cycles. By maintaining the limiting stresses below the endurance limit, fatigue failure of a material may also be prevented.
One way of trying to prevent fatigue failure of a threaded component, such as a bolt or stud, is to change the geometry of the threads on the nut to which it mates. Prior art changes in the geometry of the threads of a mating nut have resulted in some changes in the fatigue life of the mating component. However, changing the thread geometry may not be acceptable to a consumer because threads of new geometry may not have developed industry acceptance and may not meet other accepted standards.
As described above, it is particularly important to prevent failure of mechanical fasteners that are utilized in the aerospace industry because fatigue failure is generally characterized by sudden failure without warning. This invention relates to enhanced fatigue nuts and to improved methods for connecting threaded mating components with the enhanced fatigue nuts to reduce the likelihood of mechanical failure of the threaded mating components. This invention also provides improved methods for mechanically connecting threaded mating components with the enhanced fatigue nuts of the invention which reduce the likelihood of fatigue failure of the threaded mating components. This invention is also related to increasing the fatigue life of a mating component without changing the geometry of the threads of a nut.